1. Field of the Invention
The present invention relates to PON (: Passive Optical Network), and more particularly to DBA (: Dynamical Bandwidth Allocation) in PON.
2. Description of the Related Art
In accompaniment with the rapid development of optical-access-network technologies, the PON technology has widely been applied due to its wide bandwidth, high efficiency, and quality-of-service (: QoS) guarantee. Conventionally, in the PON system, a downstream frame has been broadcasted, and a certain specific identifier is checked to perform filtering of the downstream frame. Here, in an OLT (: Optical Line Terminal), an upstream frame is allocated to each user based on a bandwidth request from each user and SLA (: Service Level Agreement). Moreover, in the OLT, algorithms used for the bandwidth allocation may be DBA (: Dynamical Bandwidth Allocation) algorithms as well as SBA (: Static Bandwidth Allocation) algorithms.
However, when the bandwidth-allocating algorithms are the SBA algorithms, a fixed bandwidth is allocated to each ONU (: Optical Network Unit). This fixed allocation gives rise to occurrence of a waste of the communications band. Accordingly, in the actual system, research and application are widely carried out on the DBA algorithms to obtain flexible bandwidth allocation and bandwidth statistical gain.
In some of the DBA algorithms, however, the bandwidth is allocated to one or a few “busy users” unlimitedly (or in a manner of being limited to the line rate) up to an extent at which the system performance is degraded significantly. For example, in the conventional technology of ITU-T G. 983. 4 Appendix I, the bandwidth is allocated by calculating a transmission-await data amount on link route of PON. The following steps are periodically executed with respect to each communication flow:
1. Each ONU monitors queue length of each communication flow stored in a buffer. Simultaneously, the OLT calculates bandwidth allocation based on queue information that the OLT had received before that, then transmitting the bandwidth allocation;
2. Each ONU receives the bandwidth allocation from the OLT;
3. Each ONU transmits data and the queue-length information based on the bandwidth allocation that each ONU has received.
The bandwidth-allocating algorithm at the above-described step 1 further includes the following steps:
1.1. allocating a fixed bandwidth;
1.2. allocating an assured bandwidth;
1.3. allocating a non-assured bandwidth to all of “congestion users” in proportion to the assured bandwidth of each user;
1.4. checking and assuring that the bandwidth allocated has not exceeded an individual bandwidth limitation;
1.5. allocating a best-effort bandwidth finally.
At the steps 1.2 and 1.3, the unused bandwidth is reused at the next step. Namely, one user's assured bandwidth may also be reused as another user's non-assured bandwidth or best-effort bandwidth. At the step 1.4, the bandwidth allocation to each user is limited by the individual bandwidth limitation. This mechanism allows prevention of the resource's excessive occupation by the “busy users”. In the case of a group-based service (e.g., video conference), however, service amount of a particular user is large within the group in many cases. Namely, it is unlikely that all of users within the group are the “congestion users”. Accordingly, in the conventional bandwidth-allocating methods where the group factor is not taken into consideration, the bandwidth-allocating efficiency is not high.
Also, in the conventional DBA mechanism, since each ONU can transmit only the bandwidth request to the OLT, the OLT cannot acquire information other than the queue-length information from each ONU. Consequently, the bandwidth contract for each user must be set in advance, or must be set from the OLT. As a result, the low efficiency cannot be avoided in the case where a service change occurs on the user side.